During earth moving operations, such as in the strip mining of coal, large volumes of rock overlying the coal seam (the overburden) are removed to expose the seam. This land perturbation can impact the environment with mine discharges that are laden with suspended sediment, dissolved solids, heavy and minor metals and elements and, in certain cases, acidic water drainages. Within a particular strip mine, numerous strata of varying mineralogies are disrupted by the mining process and the quality of drainage emanating from the mine site is a blend of water chemistries produced by each rock type.
Acid mine drainage is an extremely acidic, iron sulfate rich drainage that forms under natural conditions when certain coal seams are mined. The acidity in such mine drainage water is produced by oxidation of certain sulphur compounds occurring in the overlying strata which are exposed to the atmosphere. Sulphur is coal and the overburden can occur as organic sulphur, pyritic sulphur or sulphate sulphur. Organic sulphur is that component which is organically bound within the organic matter of the strata and is generally not chemically reactive under weathering conditions. Sulphate sulphur usually represents the water-soluble weathering products of the disulfides, and in most cases constitutes a very small percentage of the total sulphur content that is measured in a geologic section. Pyritic sulphur is that sulphur which is found in the disulfide phase, usually as either marcasite or pyrite.
Iron sulfides, occurring primarily as marcasite or pyrite in the coal and overlying strata, which become exposed to the atmosphere and oxidize in the presence of humidity and oxygen, form soluble hydrous iron sulfates. Subsequent natural water movement dissolves these compounds which hydrolyze to produce highly acidic water drainages with high concentrations of iron and sulfate.
Chemical reactions explaining the oxidation of the iron disulfide and the generation of acidity are given by the following equations: EQU FeS.sub.2 (s)+7/2O.sub.2 +H.sub.2 O=Fe.sup.++ +2SO.sub.4.sup.= +2H.sup.+( 1) EQU Fe.sup.++ +1/4O.sub.2 +H.sup.+ =Fe.sup.+++ +1/2H.sub.2 O (2) EQU Fe.sup.+++ +3H.sub.2 O=Fe(OH).sub.3 +3H.sup.+ ( 3)
The ferrous iron generated in the reaction described in equation (1) can be further oxidized to the ferric state in accordance with equation (3) and generate additional amounts of acidity. The ferric hydroxides associated with the chemical reaction in equation (3) impart the red and yellow-orange color that is characteristic of acid mine water drainage. Such acid drainage can greatly impact and undesirably alter the natural environment and alter the ecological balance of the surrounding areas in which earth moving operations are conducted.
Calcium and magnesium carbonate (calcareous) materials appearing in various rock types also have a dominating influence on rock weathering characteristics. However, unlike the acid-forming reactions, the geochemistry of alkaline production from calcareous material is constrained by the low solubility of the calcareous minerals in water. As a result, water in contact with calcareous materials produces alkalinity concentrations that are fixed by the partial pressure of carbon dioxide (pCO.sub.2), the time of water contact, and the solubility constant of the specific mineral or rock, e.g., calcite, dolomite, limestone. This relationship, using calcite as the mineral, is expressed by the following equation: EQU log Ca.sup.++ (mg/liter)=2.56+0.362 log pCO.sub.2
and for every mole of calcium produced there are two moles of bicarbonate (HCO.sub.3.sup.-) alkalinity. Under natural conditions, the concentration of alkalinity of various rocks exposed to weathering conditions determines the level of acidity that can be absorbed and neutralized by the water system before it degrades to acid conditions. However, since the geochemistry of alkaline production is constrained by the low solubility of the calcareous material in water, the alkalinity concentrations produced in waters in contact with the calcareous materials vary with the time of contact and the aforementioned solubility constants of the specific materials with the waters.
Recent federal legislation enacted in connection with mining operations requires the submission of certain environmental resources information, including hydrological and geological data, for the permitting of mining operations. Included within such federal rules and regulations is a required submission of chemical analysis of the strata within the overburden of the proposed mine plan areas. Samples are usually taken from test borings and core samples, and are used to identify, at a minimum, those rock types which contain potential acid forming, or alkalinity producing materials. Federal regulations further require submission of a description of probable hydrological consequences of the proposed mining activities under expected seasonal conditions in the mining area, to predict the acid-forming characteristics of the water drainage from the area. Backfilled materials are required to be placed so as to minimize contamination of ground water systems with acid or otherwise harmful mine drainage, and to control or prevent discharge of acid, toxic or otherwise harmful mine drainage waters into adjacent water systems.
Thus, in the protection of the ecology of areas of land perturbation, the identification of potentially alkaline and acid producing strata is of central concern in mine permitting, mine planning and operation, and mine reclamation. With the capability of such identification, cores of overburden rock can be examined and the weathering characteristics of the various rock types assessed before mining to ascertain the quality of mine drainage that can be expected from the mining operation. In addition, the occurrence and location of potentially acidic and alkaline materials in the overburden can be identified and selectively handled during day to day operation of the mine. Further, in the course of reclaiming a mine, strata identified as potentially acidic can be relocated in lowermost layers of the replaced overburden so as to reduce their potential for creating acid water drainage.
Within the past decade, a variety of techniques have been developed which attempt to either predict a rock's weathering behavior, or anticipate the total acid or alkaline load of a rock calculated from its calcareous or pyritic sulphur content. As aforementioned, under natural conditions, the rate of release of alkalinity and acidity varies to a great extent, such that extended periods of time (weeks to months) would be required to fully assess the exact chemical weathering attributes of a particular stratum under natural weathering conditions. It has been proposed to simulate the effects of weathering characteristics on exposed rock strata by an accelerated leaching technique wherein samples of rock types from an area of interest are placed in inert chambers and subjected to a continuous flow of humidified air. Periodically, each sample is flushed with an aqueous medium and the effluent analyzed for various components indicative of certain reactions. The components analyzed were the acitidy, pH, alkalinity, and calcium, magnesium and sulphate ion concentrations. By placing the sample in an oxidizing environment and periodically flushing the sample, the field conditions of normal atmospheric oxidation of the samples with occasional flushings by rainfall may be simulated. Such a technique is described in our previously published articles entitled "Geochemical Factors Affecting Coal Mine Drainage Quality," Chapter 8, pages 129-148 of Reclamation of Drastically Disturbed Lands, 1978 By ASA-CSSA-SSSA, Madison, Wis.; and "Time As A Factor in Acid Mine Drainage Pollution," pages 41-50 of Papers Presented Before The Seventh Symposium on Coal Mine Drainage Research, NCA/BCR Coal Conference and Expo IV October 18-20, 1977 in Louisville, Kentucky, published by and obtainable from Bituminous Coal Research, Inc., Monroeville, Pennsylvania.
Such simulated weathering condition technique still requires an extended period of time in which to carry out the leaching operation to obtain the necessary data, and considerable laboratory equipment not readily available at mine site locations is required. Thus, such leaching techniques are not practical for field operation use to assess the weathering behavior of rock types at the mine site and their projected impact on mine drainage quality.
Certain prior art techniques have attempted to accelerate sulfide oxidation by the use of either strong oxidants, such as peroxide, or iron-catalyzing bacteria, to measure the amount of acidity that can be expected. Although such information may be useful in certain mining situations involving high grade igneous and metamorphic sulfide deposits, wherein the expected acid concentrations can be used for lime treatment requirements of the waste material and serve as a guide for treatment scheduling, the accelerated technique neglects the dimension of alkalinity and its neutralizing effect on ultimate acid mine drainage characteristics; therefore, such technique is generally unreliable in the assessment of the sedimentary rocks in coal mining regions.
Another prior art technique that purportedly evaluates the acid/base account of various overburden materials to predict the type of drainage that may be expected to occur with time, involves the steps of chemically digesting a pulverized portion of a rock sample to measure its pyritic sulphur content. Such sulphur content is then stoichiometrically related by equations (1)-(3), above, to the total amount of acidity that the rock can produce. Another portion of the same sample is digested in a hot hydrochloric acid solution and the amount of acid neutralized by the sample is determined by back titration with a standard base. This acid uptake is related to the total amount of alkalinity that the rock can produce. Balancing the "acid" and "base" totals determines whether an excess of acid or base exists, and supposedly predicts the chemical weathering attributes of the rock sample. However, because of the natural carbonate solubility constraint, as aforementioned, coupled with a partial pressure of carbon dioxide, an equilibrium condition is established which fixes the maximum amount of alkalinity that can be derived from calcareous material, whereas, conversely, the sulfide weathering product has almost infinite solubility. As a result, the kinetics of acid formation are radically different from those of alkalinity formation, and a serious error can be made when the rock's capacity to directly produce acidity or alkalinity is related, on a one to one basis, to the predicted concentrations which may occur within an aqueous drainage system.
In terms of predicting a rock's chemical weathering attribute, a similar error is made in the above technique in balancing the acid-base account of a particular rock unit. This fundamental error, which occurs because of the difference in kinetics of the acidity alkalinity formation, makes the technique unreliable for predicting chemical weathering attributes of various rock types. In addition, hot acid digestion of a rock breaks down the clay minerals and dissolves all siderite (iron carbonate, which will produce acid if the iron released oxidizes from the ferrous to the ferric state) and appears as part of the "base" account in the analysis. Since these components of the rock do not produce alkalinity under natural conditions, the technique is misleading when it measures them as alkaline producers.
Thus, a major problem which still exists in overburden analysis (as to whether rock will produce acidity, alkalinity or remain inert), centers about the ability to perform rapid and accurate determinations of parameters that predict long term weathering characteristics of rocks.